A typical configuration for a conventional high-power optical amplifier is shown in FIG. 1. By way of illustration, amplifier fiber 10 may be a cladding-pumped multimode or slightly multimoded fiber co-doped with erbium and ytterbium (an “EY fiber”) for pumping in the band near 940 nm and for producing optical gain in the 1550-nm band. In the configuration shown, tapered fiber bundle (TFB) 20 couples pump light from pump lasers 30 and 32 into fiber 10, and TFB 25 likewise couples pump light from pump lasers 35 and 37 into fiber 10. Each TFB has a typically undoped, single-mode center fiber (22 and 27, respectively) coupled to fiber 10 by a respective splice 24, 29. As seen in the figure, a low-power input optical signal on fiber 40 enters the amplifier from the left, and an amplified high-power output signal exits the amplifier on fiber 45 at the right. It should be noted that although, for simplicity, the figure shows only two pump lasers connected to each TFB, it is more typical to use a greater number, such as six pump lasers per TFB.
In operation of the amplifier of FIG. 1, low-power signal 40 combines with pump light as it passes through TFB 20, undergoes amplification in fiber 10, passes through TFB 25, and exits as high-power signal 45. Fiber 10 typically has a core that is 10 microns in diameter and a cladding that is 125 microns in diameter. The signal light is typically guided in one or more core modes of fiber 10, whereas the pump light is typically guided in one or more cladding modes. A low-index outer cladding, commonly of a polymeric material, is often used to help confine the pump light.
Optical amplifiers of the kind described above suffer from parasitic lasing. For example, EY fibers provide gain not only in the 1550-nm signal band, but also in the 1060-nm band. If 1060-nm light experiences sufficient gain in a pass through an optical cavity, and further experiences sufficient reflectivity at the ends of the optical cavity, undesired lasing may take place in the 1060-nm band. This lasing often takes place in higher-order modes of the EY fiber. Such lasing is undesirable, not least because in high-power operation, it can damage optical components.
In fact, parasitic lasing has been observed in high-power EY fiber amplifiers. The conditions for parasitic lasing may be due, for example, to high values of the optical gain per unit length, and relatively high reflectivity at interfaces such as the exit windows of the pump lasers. Consequently, the need to avoid such parasitic lasing may limit the operating power levels of optical amplifiers. Greater power levels could be achieved, in at least some cases, if such parasitic lasing could be suppressed.